@CaptainDangernoodle, there is a way around this issue with the exchange rate. You can invest in currency-hedged ETFs to avoid speculating on the future exchange rate.
GBP-Hedged ETFs, as I understand it, periodically hedge the currency to avoid variations in the exchange rate affecting performance. ie. You get the return of the Index without considering changes in the exchange rate with the othe currencies, in this case the Dollar.
This is good if you think that in the long term the Pound will appreciate (strengthen) against the other currency eg. Dollar or Euro. On the other hand, if you think it will depreciate (weaken) buying the un-hedged (normal) ETF is better as an appreciation of the dollar will give you more pounds.
My plan is** to go 75% hedged ETFs (avoiding currency exchange variations) and 25% unhedged (normal) on USA ETFs, as I already have some other un-hedged World and Value ETFs as well as US stocks which will get exposure to the Dollar directly. So overall only around 30% of my USA exposure would be hedged.
Also to note: Hedged ETFs tend to have slightly higher OCFs than their unhedged equivalents.
If you are interested in GBP-Hedged ETFs to reduce exposure to the exchange rate on the pound, particularly now that it is quite weak due to Covid-19 and Brexit, I requested some in this post. They have not been added yet, so please support them if you are interested in them being added 
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**Note: The plan is not currently in place as there aren’t any GBP-Hedged ETFs currently.